1. Technical Field
The present invention relates to a communications network and to a method and device for retiming a data pulse carried on such a network, in particular for correcting for timing fluctuations.
2. Description of Related Art
It is known that optical fibre has a huge potential information-carrying capacity. For example, by utilising the entire gain bandwidth of erbium-doped optical amplifiers, a single fibre could carry more than 2 Tbit/s. However in the majority of telecommunications systems in commercial use currently, the information is carried over fibre in the form of an optical signal at a single wavelength. The data transmission bandwidth of the fibre is therefore limited by the electrical bandwidth of the transmitter and receiver, and this means that only a tiny fraction (a maximum of about 1%) of the potential bandwidth-carrying capacity of the fibre is being usefully exploited. There is therefore much interest currently in developing methods for increasing the transmission rate for point-to-point fibre links. One method is wavelength-division multiplexing (WDM), in which several data channels, at different wavelengths, are carried simultaneously on the same fibre. An alternative method for increasing the rate of information that can be carried on fibre is to use optical time-division multiplexing (OTDM) in which several data channels are multiplexed in the form of bit-interleaved return-to-zero (RZ) optical pulse trains.
The WDM approach to photonic networking has some very attractive advantages: in addition to the relative simplicity and commercial availability of the devices needed, WDM networks can be created in a wide variety of architectures with great flexibility (the main restriction being merely that any pair of photonic transmission paths cannot use the same wavelength on a shared fibre link). An advantage of WDM networks is that they can, in principle, support ‘signal transparency’, i.e. data signals can be carried using any modulation format. However, this implies that, in effect, WDM photonic networks are based on ‘analogue’ transmission. As a result it is not possible for digital signal regeneration techniques in the optical domain, to be used. The inability to perform signal regeneration in the optical domain leads to practical scaling limitations for WDM networks due to noise accumulation from optical amplifiers, crosstalk and nonlinearity. These factors restrict the number of network switching nodes through which signals can pass without fatal degradation. Currently, in reported laboratory experiments the maximum number of WDM switching nodes through which a signal can pass without regeneration is limited to around 10, which is a significant restriction in architecture and scalability. A feasible, though costly, solution currently being advocated by some equipment vendors is to sacrifice transparency, standardise the transmission format, and regenerate each wavelength channel individually at the outputs of WDM cross-connects. In effect, this is a hybrid arrangement using analogue switching together with channel-by-channel digital regeneration.
In the OTDM approach to photonic networking, the signals are carried in ‘digital’ format in the form of RZ optical pulses, allowing the use of digital signal regeneration techniques in the optical domain such as 3R (Re-amplify, Re-time and Re-shape) regeneration [Lucek J K and Smith K,Optics Letters, 18, 1226–28 (1993)] or soliton-control techniques [Ellis A D, Widdowson T, Electronics Letters, 31, 1171–72 (1995)]. These techniques can maintain the integrity of the signals as they pass through a very large number of nodes. For example, Ellis and Widdowson [Ellis A D, Widdowson T, Electronics Letters, 31, 1171–72 (1995)] have made a laboratory demonstration of error-free transmission of signals through an OTDM network consisting of 690 nodes in concatenation. Despite this impressive potential for scalability, however, the OTDM approach to photonic networking suffers from severe restrictions in the network architecture that can be used. This results from the need to maintain proper bit-level synchronism between all the signal sources, demultiplexers and channel add/drop multiplexers throughout the network.
The problems with the conventional techniques discussed above, are that in complex architectures, timing fluctuations of the data pulses in the arrival time of pulses at nodes (due to environmental effects acting on the fibres such as temperature change and mechanical strain) cannot be adequately controlled or compensated in a continuous uninterrupted fashion. This results in data pulses being lost. There are many causes of timing fluctuations that may result in data being lost. The first cause is jitter in the arrival time of the incoming packet data pulses. It is known that in high-speed optical transmission systems, jitter in the arrival time of pulses arises from effects such as amplified spontaneous emission noise, the soliton self-frequency shift arising from the Raman effect, soliton short-range interactions, and the complex interplay of these various processes. Other timing fluctuations include temperature dependent length changes in the fibre that cause the absolute arrival time of the optical pulses at a node to wander. This creates timing problems for demultiplexing the data at the node and for adding new local data to the optical stream. Techniques to provide synchronism at nodes and overcome this wander timing problem have been described using discrete wavelength conversion and dispersion compensation (K. S. Jepsen et al, Technical University of Denmark, ECOC '97 postdeadline) but this technique requires feedback to achieve synchronism. The limited bandwidth of such feedback control limits this technique to relatively slow timing changes and not pulse-to-pulse jitter.
Conventional techniques to compensate for timing fluctuations, such as jitter, rely upon a gate window being opened by the timing pulse when it reaches the node. The problem with this technique is that the gate window only has a finite duration, and if a data pulse is affected by jitter to the extent that it does not arrive within the gate window, it will be lost.